In order to improve display performance of image display devices such as a liquid crystal display device and an organic EL display device, various types of phase difference films have been utilized. Among them, a phase difference film having an in-plane retardation that becomes greater toward a longer wavelength side (hereinafter, referred to as a “reverse wavelength dispersion film”) can be used as an antireflection layer for a reflective-type liquid crystal display device a touch panel and an organic EL display device.
A reverse wavelength dispersion film to be used as an antireflection layer is preferably such that (i) a phase difference is approximately one-fourth of a measurement wavelength λ, and (ii) a ratio Re(450)/Re(550) between an in-plane retardation at 450 nm and an in-plane retardation at 550 nm is close to 0.81. Further, in order to achieve a thickness reduction of display devices, a small film thickness of approximately 20 μm to 50 μm is demanded of a reverse wavelength dispersion film to be used. Thus, various types of phase difference films have been developed to respond to such a demand for the above described properties.
Patent Literature 1 discloses a reverse wavelength dispersion film that is composed of cellulose acylate and ethyl cellulose. The phase difference film disclosed in Patent Literature 1 has an in-plane retardation of approximately λ/4 and is suited for an intended use as an antireflection layer. However, the reverse wavelength dispersion film disclosed in Patent Literature 1 faces the following trade-offs. That is, an improvement in reverse wavelength dispersion property causes an increase in film thickness. Conversely, a reduction in film thickness causes a reverse wavelength dispersion property Re(450)/Re(550) approaching 1. Thus, the reverse wavelength dispersion film disclosed in Patent Literature 1 requires a further improvement to satisfy both a film thickness and a reverse wavelength dispersion property.
Patent Literature 2 discloses a reverse wavelength dispersion film that is composed of cellulose acylate and ethyl cellulose. In order to use the phase difference film disclosed in Patent Literature 2 as an antireflection layer, it is necessary to adjust an in-plane retardation of the phase difference film. Since an in-plane retardation is proportional to a film thickness, it is possible to achieve an in-plane retardation of approximately λ/4 by increasing the film thickness. However, increasing the in-plane retardation by adjusting the film thickness increases the film thickness. Therefore, the reverse wavelength dispersion film of Patent Literature 2 requires a further improvement.
Patent Literature 3 discloses a reverse wavelength dispersion film that is composed of cellulose acylate and ethyl cellulose. The phase difference film disclosed in Patent Literature 3 is higher in phase difference exhibition property than those disclosed in Patent Literatures 1 and 2. Thus, the phase difference film of Patent Literature 3 is capable of, while having a small film thickness, exhibiting the same in-plane retardation as those of the phase difference films disclosed in Patent Literatures 1 and 2. However, the phase difference film of Patent Literature 3 still has a film thickness of approximately 60 μm to 70 μm, and thus requires a further improvement.
Patent Literature 4 discloses a cellulose derivative that has, as substituents, various types of aromatic acylates and aliphatic acylates that differ from each other in maximum absorption wavelength and molar absorption coefficient. The phase difference film disclosed in Patent Literature 4 is capable of exhibiting an intended reverse wavelength dispersion property, but has a considerably low phase difference exhibition property. Thus, in order to achieve an intended in-plane retardation, it is necessary to increase a film thickness of the phase difference film to approximately 80 μm. Therefore, the phase difference film of Patent Literature 4 requires a further improvement.
Patent Literature 5 discloses a cellulose derivative film having a great film thickness direction retardation (Rth). A material of this film includes aromatically esterified cellulose ether. However, Patent Literature 5 does not employ a processing technique such as heat stretching, and Patent Literature 5 makes no mention of the in-plane retardation and the reverse wavelength dispersion property that are essential properties for the present invention. The film disclosed in Patent Literature 5 is a VA-type liquid crystal compensation sheet for which an unstretched film produced by solution casting is directly used, and has properties different from the properties required of the present invention. Thus, the film disclosed in Patent Literature 5 has a film thickness that is approximately twice larger than that of the present invention.